The present invention relates generally to the field of vehicle hydraulic systems. The present invention relates more specifically to a load sensing system for vehicle hydraulic system including multiple hydraulic pumps and multiple actuators.
Hydraulic systems typically include a pressure source (e.g., a hydraulic pump), a hydraulic circuit through which the pressurized fluid is transported, and one or more devices (e.g., hydraulic cylinders, hydraulic motors, etc.) in which the pressure is used to do work. Flow of hydraulic fluid to the device may be controlled with a valve in the hydraulic circuit. The hydraulic pump may be a fixed displacement hydraulic pump that provides a fixed flow rate of hydraulic fluid. Because the hydraulic system may include several devices and more than one of the devices may be operated simultaneously using the pressurized hydraulic fluid, the pump is typically sized to provide a flow rate and pressure that is sufficient to run all or a number of the devices simultaneously. However, if only one device is operated with the hydraulic pump providing an excessive flow rate and pressure, excess pressure accumulates in the hydraulic circuit. To prevent damage to the components of the hydraulic system, a high pressure relief valve may be provided in the hydraulic circuit which allows excess hydraulic fluid to pass through the system (e.g., to a fluid reservoir). However, the hydraulic pump consumes excessive energy to provide a high fluid flow rate, the excess energy being converted to heat that can shorten the system life and jeopardize system function.
In current refuse industry vehicles, the hydraulic systems may include multiple hydraulic pumps working together to provide the pressure and flow rate required by several devices such as hydraulic actuators, with the flow to each of the devices controlled through valves in the hydraulic circuit. The hydraulic source used may be variable displacement hydraulic pumps with a pressure compensating load sensing system to regulate the output of the source and provide hydraulic fluid at a desired flow rate and pressure to match the varying demand of the multiple devices. The load sensing signals, the flow, or both are typically combined before the control valves.
When performing refuse collection functions, the working load faced by one valve from the associated actuator will likely affect both pumps. If a high load is sensed at one valve, the pressure in the system is increased to compensate and the other valves operate under unnecessarily high pressures, reducing the energy efficiency of the hydraulic system and potentially causing undesired, erratic movement of the actuators. In some cases, the unnecessarily high pressure can prevent normal performance of the system or contribute to the premature failure of components of the hydraulic system. When the load sense and flow are combined between pumps and the valves, one pump may become saturated before the other pump begins operating. One pump may work constantly while another pump only works intermittently. Flow limiters may be added to prevent one pump from operating excessively, but this adds cost and complexity to the hydraulic system. Further, the combined load sensing typically may amplify the load shocks and pressure spikes in the hydraulic system. This amplification can cause erratic or jerky movement of the actuators and accelerate premature failure of the components of the hydraulic system.